This invention relates to engine exhaust emissions measurement methods and equipment, and more particularly, the invention relates to a method and apparatus for correcting sample exhaust gas flow through a mini-diluter.
Accuracy in taking engine exhaust emission measurements has become increasingly important in recent years in view of more stringent vehicle emission standards. Permissible emissions under these standards are very low such that the accuracy currently acceptable emission equipment may not be sufficient to distinguish between a vehicle with acceptable emission levels and a vehicle with unacceptable emission levels.
One system that is frequently used to test emissions is referred to as a mini-diluter in which the exhaust emissions are diluted to a lower sample concentration and then a portion of the sample is either analyzed online or stored in a bag for analysis. The dilution unit must be calibrated so that the exhaust emissions are diluted to obtain a dilution ratio that must remain constant throughout the test. In particular, the flow of a dilution gas (diluent), such as nitrogen or synthetic air, and the exhaust gas is set to obtain a desired dilution ratio in order to avoid water condensation in the sampling system. Typically thermal mass flow controllers are used to control the flow of the diluent and raw exhaust gas. Due to their measurement principle thermal mass flow controllers or meters show a strong dependency of the reading on the chemical composition of the measured gas. However, the exhaust gas has a different density and specific heat than the diluent such that when exhaust gas flows through the metering device in the dilution unit an incorrect dilution ratio is obtained. As a result, the sample collected in the bag does not accurately represent the exhaust gas emissions expelled from the vehicle and an inaccurate result is obtained.
The prior art has masked this inaccuracy by adjusting the amount of sample collected in the bag rather than adjusting the flow rate of exhaust gas emissions to obtain the correct dilution ratio. Data collected on the exhaust gas flow rate is therefore inaccurate. Increased accuracy may be necessary for post-analysis. Another common method is to calibrate the exhaust gas mass flow controller with a mixture of CO2 and Nitrogen; this method does not account for changes of the exhaust gas composition and for the effects of the water content.
Partial flow particulate samplers, which measure the particulates in diesel emissions, also utilize mass flow controllers that are calibrated in a similar manner to that of mini-diluters. As a result, partial flow particulate samplers may also have inaccuracies. Therefore, what is needed is an engine exhaust emission measurement correction that yields a correct dilution ratio.
The present invention provides an exhaust emission analysis system including a dilution gas source respectively providing dilution gas. A dilution unit includes exhaust and dilution gas flow devices, such as mass flow controllers, fluidly connected to the exhaust and dilution gas sources, respectively. The metering devices in the mass flow controllers define a gas flow rate of gas from its respective gas source. The gas flow devices are fluidly connected at a connection that mixes the gases to provide a diluted exhaust gas having an incorrect dilution ratio if no further adjustment was performed. A humidity measurement device such as an analyzer measures the water content of the exhaust gas, preferably subsequent to dilution. A humidity content signal corresponding to the water content in the exhaust gas is sent from the water measurement device to a control device. The CO2 content of the exhaust gas may be calculated using the chemical analysis of the fuel and the air/fuel ratio or by directly measuring the CO2 content. An adjustment factor is calculated by the controller and a flow rate command signal is sent from the controller corresponding to the adjustment factor, preferably, to the exhaust gas mass flow controller to adjust the gas flow rate of the exhaust gas and provide a corrected dilution ratio at the connection.
The present invention also provides a system having a particulate sampler that includes a probe for proving exhaust gas. The particulate sampler has a mixer introducing the dilution gas to the exhaust gas to produce diluted exhaust gas, which has an uncorrected dilution ratio. Diluted exhaust and dilution gas flow devices are fluidly connected to the diluted exhaust and dilution gas sources respectively. The flow devices define a gas flow rate of gas from its respective gas source. Similar to the exhaust emission analysis system described above, a water content and/or carbon dioxide content may be determined which may be used in calculating an adjustment factor to provide a corrected dilution ratio at the mixer.
Accordingly, the above invention provides an engine exhaust emission measurement correction that yields a correct dilution ratio.